Embossed absorbent paper sheet

ABSTRACT

The invention concerns a web of absorbent paper, in particular of cellulose cotton, including on one side protrusions of which the top surfaces are less than 1 mm 2  each and which correspond to cavities on the other side, the web being characterized in that the protrusions are distributed at a rate of more than 30/cm 2  along mutually parallel lines in a manner such as to bound cells of which the total surface takes up at least 30% of the web surface, the cells per se being unembossed or embossed only slightly.

The present invention relates to absorbent papers for sanitary orhousehold use and its objective is a web composed of at least oneembossed ply to make toilet paper foremost, but also handkerchiefs,napkins or paper towels.

In order to manufacture this kind of paper, the sanitary and householdpaper industry uses absorbent paper which generally is creped and of lowweight density, called cellulose wadding or cotton wool fabric. Thestructure's stretchability, for example, imparted by creping, is used toemboss the web. This processing permanently deforms the web between anundeforming cylinder having a topology of salients and a matingcylinder, illustratively, coated with a resilient material. In thismanner, protrusions are made on one side which correspond to cavities onthe other side.

The trend in recent years for sanitary products made of cellulosewadding has been to make them softer, more velvety, by operating ontheir thickness and mechanical strength, in particular by embossingthem. Embossing moreover allows improving product appearance. Embossingis carried out either on substantially wet paper, that is in the wetportion of the papermaking machine, or on low-moisture paper, when beingmechanically worked. In that case, work is performed on a dry webunwound from a mother spool of the papermaking machine. The presentinvention concerns paper being worked in the dry part.

The most widely used embossing patterns are geometrically-based repeatsof elementary protrusions of small cross-sections and simple geometricshapes. U.S. Pat. No. 3,414,459 illustratively describes a stratifiedweb consisting of a plurality of elementary plies which are bonded toeach other. The plies therein are embossed at a distribution rate and ata protrusion height designed for water-absorbing products, for examplepaper towels. The element density is from 5 to 30/cm². Applicant on itspart has developed, in particular as regards toilet paper, patterns ofwhich the element density is higher, ranging from 30 to 80/cm². Perforcethe tops of these elementary surfaces are minute, each being less than 1mm². Such products offer an appearance imitating that of a woven fabric.European Pat. No. 426 548 illustrates such an embodiment. This kind ofembossing, which is fine and dense, is well suited to impart theappearance of thickness to the web.

However, its appearance is modest. Moreover, when two plies embossed inthis manner are joined together, product softness is less than optimal.

French Patent Application No. 94 15196 offers a solution in improvingthe softness of a double-ply web made with this kind of embossing byusing a joint pattern including a background pattern and a graphicpattern. The latter is composed of linear protrusions of a width between0.1 and 2 mm and the former includes small and generally frustoconicalprotrusions distributed at the rate of at least 30/cm². The plies arecombined on the linear pattern, thereby limiting the surface of themutually glued surfaces and the ensuing stiffness.

The background pattern protrusions must bring out the main pattern sincethey are not being easily seen with the naked eye on account of theirinherent small size. However, during manufacture, the graphic pattern,which is salient on the cylinder, comes to rest against the rubber ofthe embossing cylinder just as do the teeth of the background pattern.Therefore the two kinds of embossing compete with each other. Due totheir small size, the teeth of the background pattern are more effectivein embossing than those of the graphic pattern and consequently thelatter will leave a less defined imprint.

On the other hand, if a better defined graphic pattern is desired, therequired embossing conditions will degrade the web properties, inparticular its mechanical strength, both at the edges of the patternitself and in the base texture. Another consequence is degradation ofmanufacturing conditions, for example degradation of the rubbercoatings.

Furthermore, the proportion of the embossed surface of the backgroundpattern is predominant, namely being 80% of the surface in practice.This feature ensures the main operational characteristics relating toembossing, namely thickness and absorption. During manufacture, the highpressure between the engraving and the mating rubber surface will bespread over a large number of elements. As a result, local stresses andthe danger of tearing the web are reduced. Moreover, the pressure isapplied substantially isotopically to the web, thereby limiting thedanger of creating rupture paths.

Moreover, it is known that if the density is reduced, the stresses willbe concentrated and the web is commensurately weakened.

The objective of the invention is a web of embossed absorbent paperoffering a more elaborate pattern than a plain screen pattern withoutusing linear protrusions.

Another object of the invention is to ensure homogeneous marking andgood pattern definition.

Another objective of the invention is a two ply or three ply web ofembossed absorbent paper of which the embossing pattern provides optimalpreservation of flexibility following combination.

Another objective of the invention is a web of embossed absorbent paperwith a thickness and mechanical strength roughly the same as in a web ofwhich the embossing is uniformly distributed and uniformly higher than30 protrusions/cm².

In the invention, a web made of absorbent paper, in particular of crepedcotton wool and of a specific surface weight between 10 and 40 g/m² andincluding at one surface first protrusions having a peak area less than1 mm² and corresponding to cavities on the other surface, ischaracterized in that the protrusions are distributed at a rate of atleast 30/cm² along mutually parallel lines in a manner such as to boundcells of which the total surface takes up at least 30%, preferably atleast 50%, of the web surface, these cells per se being embossed littleor not at all.

A cell is a portion of the web surface enclosed by parallel linessubtended by the first protrusions and themselves including 30% ofuntextured zones. The cell shapes are arbitrary, though preferably theyare geometric. The lines defining them may mutually cross to subtendgrids, although the cells also may be disjointed.

Surprisingly, it was discovered that a web with embossing so definedoffers the same thickness and mechanical strength as a web having anembossing composed of the same elements uniformly distributed over itssurface at a higher uniform density. This feature is especiallysignificant when making a web of several plies embossed in this mannerbecause the non-uniform distribution allows partially bonding the pliesto one another. Greater flexibility is obtained as a result.

In addition, cumulative perception sets in on account of the pattern'sdistribution rate. Softness is improved because of the unembossed oronly slightly (partially) embossed surface of the cell of which the sizenevertheless is adequate to carry out calendering at the time ofembossing.

Calendering arises from the action by the rubber of the mating cylinderon the web when this rubber presses against the inter-tooth cylindersurface. Calendering makes the web smoother by eliminating roughness. Asa result, the perception of softness is increased. Softness is enhancedfurther if the calendered zone is substantial enough to be felt whentouching it. Therefore, the solution of the invention advantageouslyprovides that the portions of the calendared surface are at least 1 cm².This magnitude corresponds to the contact surface between the web andthe fingers.

In another feature of the invention, the first protrusions aredistributed along at least three lines when the cells are adjacent. Ifthere are fewer lines, preferential rupture paths may form.

In another feature of the invention, the lines are wavy. In this manner,the danger of forming low strength paths is reduced.

In another feature of the invention, the web includes second protrusionsinside the cells. In this case, these are isolated protrusions. In otherwords, the protrusions are configured to subtend sub-cells large enoughfor the rubber to reach the bottom of the engraving during the embossingprocedure and thus to ensure that the web is be calendered at that site.Advantageously these second protrusions are arrayed in unit linesrunning parallel to the cell edges. These protrusions shall never takeup more than 15% of the cell surface.

In another feature of the invention, the first protrusions define atleast 5 cells per segment, preferably 15 to 20. Preferably, the secondprotrusions among themselves, or between themselves and the firstprotrusions, define at most 100 sub-cells. A segment is the web portiontorn off along a transversely pre-cut line, for example in a roll oftoilet paper.

In another feature of the invention, the cells are not adjacent. Inparticular the space between the cells then may be taken up by atextured zone. This textured zone then preferably has an embossingdensity different from that of the first protrusions.

In another feature of the invention, the web constitutes at least oneply of a sheet of several plies. Due to this design, the cellconfiguration of the protrusions allows partial bonding and the makingof a more flexible multiply product.

An illustrative and non-limiting embodiment of the invention iselucidated below in relation to the attached drawings.

FIG. 1 is a top view of a web embossed in the manner of the invention,and

FIG. 2 is a plot of the relation between mechanical strength andthickness for a product of the prior art and a product of the invention.

The illustrated web is a creped cellulose cotton web having a specificsurface weight between 10 and 40 g/m². The dots represent protrusions onone side and cavities on the opposite one. These protrusions were madeby deforming the web between a rigid surface component, such as acylinder, and a mating component, such as a cylinder fitted with aresilient coating, for example rubber. The rigid surface includessalients configured according to a desired pattern to be imparted to thecellulose cotton web. The shape of the embossing teeth is frustoconicaland their cross-section is circular, oval or polygonal.

The pattern shown in FIG. 1 is composed of juxtaposed cells bounded bymultiple rows configured in a crossing manner. Illustratively, thediamond-shaped cell C is defined by triple protrusion rows B₁, B₂, B₃.These rows are slightly wavy.

In the invention, the first protrusions are configured so that the localdensity of the first protrusions are larger than 30/cm². Preferably itis larger than 60/cm².

Their surface at the top is less than 1 mm². To reach the goal of theinvention, namely mechanical strength and thickness equivalent to thoseof a web embossed uniformly across the full surface and having a densityof the same order, the number of rows or lines is at least three whenthey bound adjacent cells.

The inside of the cell C is unembossed or else includes unembossed zonesbounded by isolated second protrusions I. This designation applies toprotrusions subtending between themselves or between them and the celledges so-called sub-cell zones where the web is calendered. Thiscalendering is possible only if there is sufficient free space to allowthe resilient material of the mating part to press against the basesurface between the teeth. In the embodiment shown, these isolatedprotrusions subtend lines running parallel to the cell edges anddividing the cells into nine sub-cells.

Tests were run to compare the behavior of a web of the invention withthat of a web of the prior art. The Table below lists the means used andthe results.

The prior art pattern consists of uniformly distributed protrusionsconfigured, for example, in rows running substantially in the directionof advance and one row staggered relative to the next, the density being80 protrusions/cm². The surface at the top of the embossing teeth is 4.4mm². The pattern takes up 100% of the surface.

The pattern of the tested invention is that of FIG. 1. The averagenumber of protrusions is 25/cm². However, the density rises to 60/cm² inthe zones B₁, B₂ and B₃ forming the edges. The protrusions take up 20%of the web surface. In other words, the sub-cells take up 80% of thesurface.

TABLE Pattern of the Prior-art Pattern Figure Mean density 80 Teeth/cm²25 Teeth/cm² Local density 80 Teeth/cm² 60 Teeth/cm² Tooth diameter 0.4mm 0.4 mm % embossed 100% 20% surface Thickness Thickness (mm)/strengthImpression (mm) (mm)/strength (N/m) (N/m)  0  0.21/100  0.21/100 150.33/75 0.37/74 19 0.49/51 0.45/64 22 0.52/51

Teeth of the same geometry and of the same size were used for the twopatterns. The impression is the size of the rigid cylinder mark on therubber cylinder. The impression is more pronounced the higher theapplied pressure when the rubber quality is constant. Accordingly, theimpression is wider the higher the pressure. The zero value of theimpression denotes the web characteristics before embossing. By plottingthe values in FIG. 2 showing the web's mechanical tear strength as afunction of thickness, and by drawing a linear regression line betweenthe found values, the pattern of the invention is shown to offercharacteristics equivalent to those of a prior art pattern.

Accordingly, without trading away the desired features of high densityembossing, a more flexible product is achieved. It is understood that aply embossed with the claimed pattern can be combined with an identicalor different ply of the prior art to make a comparatively flexible webon account of the scarce number of bonding sites.

What is claimed is:
 1. An absorbent cellulosic web comprising a firstside having first protrusions, each first protrusion having a topsurface less than 1 mm², and said first protrusions corresponding tocavities on a second side of said web, wherein said first protrusionsare distributed at a rate of more than 30/cm² along mutually parallellines in such a manner as to bound cells, said cells taking up a totalsurface of the web of at least 30%, said cells being unembossed or onlypartially embossed.
 2. Web as claimed in claim 1 wherein the totalsurface of the cells takes up a total surface of the web of at least50%.
 3. Web as claimed in claim 1 wherein the first protrusions aredistributed along at least three lines between two adjacent cells. 4.Web as claimed in one claim 1 wherein the mutually parallel lines arewavy.
 5. Web as claimed in claim 1 further comprising isolated secondprotrusions bounding sub-cells within the cells.
 6. Web as claimed inclaim 1 wherein the absorbent web is formed of multiple segments andfirst protrusions define at least 5 cells per segment.
 7. Web as claimedin claim 6 wherein the first protrusions define from 15 to 20 cells persegment.
 8. Web as claimed in claim 5 wherein the second protrusionsdefine in themselves or between themselves and the first protrusions nomore than 100 sub-cells.
 9. Web as claimed in claim 1 wherein the cellsare not adjacent to each other.
 10. Web as claimed in claim 9 whereinspace between the cells has a texture with a density different fromtexture defining the cells.
 11. Web as claimed in claim 1 wherein theweb constitutes at least one ply of a multi-ply sheet.